How to factor the expression $1+s-t-st$

Question

How should this expression be factored $1+s-t-st$ ? I don't know how to begin & I'm clueless.

Your help would be much appreciated. Thanks

Answer 1

$$1+s-(t+st)$$ $$(1+s)-t(1+s)=(1+s)(1-t)$$

Answer 2

$$1+s-t-st$$

$$=1(1+s)-t(1+s)$$

$$=(1+s)(1-t)$$

Answer 3

\begin{align} 1+s-t-st &= 1-t+s(1-t) \\ &=(1-t)(1+s) \end{align}

Answer 4

When $s=-1$, $1+s-t-st=0$ and similarly for $t=1$. So we have either $(1+s)(1-t)$ or $(1+s)(t-1)$. By inspection, $1+s-t-st=(1+s)(1-t)$

Answer 5

$1+s-t-st$ is same as writing $1\times(1+s)-t(1+s)$.

Now take out $(1+s)$ common you are left with

$(1+s)(1-t)$